widget customizations for advance search widget

I want to modify how the content is outputted for the categories part of the advance search widget, I found the advance search widget class in DT/frameworks/class/class_widgets.php on line # 1290 but lost the trail when i saw $advance_search = Core_Advanced_Search::instance(); echo $advance_search->build_form( null, $submit );. Where can I find thisbuild_form() function, is that where I can edit the contents. Attachments show what the widget outputs for category listing by default, other attachment shows what is trying to be accomplished.